Medical applications often use surgical instruments that must fit within a physician's hand. When operated, the instrument must have constant motion and little backlash reaction. A linear actuator produces no rotary torque on startup, and a very small device which provides substantial force for doing work at multiple attitudes and positions and which allows for very accurate control would be well-received by the art.
One specific kind of linear actuator uses the shape change of piezoelectric materials that occurs when voltage is applied thereacross to generate linear motion. A "double clamping" type actuator operates by clamping a first end of the piezoelectric material, expanding the overall length of the material, clamping the other end of the piezoelectric material and releasing the first end of the material, and then reducing the size of the piezoelectric material. Each repetition of the above-described process causes one cycle of movement.
The Inchworm.TM. piezoelectric linear actuator manufactured by the Burleigh Corp. of Fishers, N.Y. is one example of a double clamping actuator. The Inchworm.TM. device includes three piezoelectric elements, two of which mechanically and orthogonally grip a shaft that axially extends through the motor. Even the smallest of these devices, however, is too large in diameter (about 0.5 inch) for many micro-device applications.
A different linear actuator, which uses only a single kind of clamping mode, is disclosed by Judy, Polla and Robbins in "A Linear Piezoelectric Stepper Motor With Sub-micrometer Step Size and Centimeter Travel Range", IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 37, No. 5, September 1990 ("Judy et al."). The piezoelectric element described therein measures 25.4 mm.times.12.7 mm.times.1.6 mm, while the overall size of the stepper is greater than 150 mm.times.60 mm.times.100 mm. The principle of operation of the device is based on the expansion and contraction of a piezoelectric element mounted on a gliding structure.
The Judy et al. design was relatively large in size, difficult to assemble, and varied greatly in performance characteristics depending on its physical orientation--the direction in which it was being held. The Judy et al. device used components made of bulk piezoelectric ceramics, metal and non-conducting insulators that made it difficult to reduce the overall size of the device. Brass glider electrodes electrostatically clamp a teflon insulating layer between the plates. The teflon has a thickness of about 63.5 .mu.m. Judy's system operated by clamping the piezoelectic element during its expansion, so that it could not move relative to the base and hence pushed the load forward. During contraction, the clamping force was removed so that the piezoelectric element would move relative to the base.
Two types of double clamping linear motors are disclosed in Fujimoto in U.S. Pat. No. 4,736,131 and Lang in U.S. Pat. No. 4,709,183. In the U.S. Pat. No. '131, two piezoelectric elements clamp on side walls perpendicular to the axis of motion through motion magnifying levers. The U.S. Pat. No. '183 also relies on the piezoelectric material to achieve mechanical clamping of the side wall to implement a double clamping linear motion. Sidewall clamping requires a high degree of manufacturing precision (orthogonal machining) and cost.
Linear motors that rely on vibratory phenomena employ several different strategies, including the use of piezoelectric elements to transfer vibrations to moving members. These types of devices are inefficient, expending substantial energy in generating motion off axis, or orthogonal, to the intended line of work. Linear, circular or elliptical motion is repetitively, frictionally transmitted to the moving members. A variety of driving methods have been used, including axial, torsional, and traveling wave vibration phenomena.
For example, Onishi's U.S. Pat. Nos. 5,036,245 and 5,134,334 describes a device in which piezoelectric elements vibrate the legs of a "C" shaped structure such that the structure travels along a rail perpendicular to the legs.
Yamaguchi's U.S. Pat. No. 5,140,215 discloses a device that transfers elliptical motion with one member exciting a longitudinal vibration in the length direction while another member excites a flexural vibration in the thickness direction. All of these devices in the prior art are unsuitable for the kind of miniaturization used according to the present invention.
Many of the problems outlined above are solved by the miniature linear motion actuator in accordance with the present invention. The linear motion actuator hereof provides for precise, controllable linear motion in a substantially miniaturized device.
A linear motion actuator for effecting incremental, bi-directional linear motion is disclosed. The linear motion actuator has a voltage supply, first and second wafer elements presenting their respective first and second wafer surfaces in slidable, abuttable engagement, a clamping system for selectively holding the first and second wafer surfaces in relative position, an expandable member operably coupled to the first wafer element to effect motion of the first wafer relative to the second wafer, and an electronic control system, electrically connected to the voltage supply, the clamping means and the expandable member being adapted for controlling various characteristics of the motion. The electronic control system controls the amplitude, frequency and waveform shape of a voltage output to the clamping system and the expandable member whereby the expandable member effects a controlled, selective bi-directional, linear motion of the first wafer relative to the second wafer.
Preferably, semiconductor technology is combined with shape changeable materials such as piezoelectric materials to provide a miniaturized linear actuator. An electrostatically clamping semiconductor wafer and base semiconductor wafer, both having polished surfaces, are placed in slidable, abuttable engagement. Wire leads selectively provide an electrostatic clamping force between the wafers. A selectively expandable piezoelectric element is fixedly carried at a first end of the clamping wafer. The second end of the piezoelectric element is coupled to an inertial mass. An actuating voltage is selectively applied to the piezoelectric element while a clamping voltage is applied across the wafers, holding the clamping wafer in position relative to the base wafer. The actuating voltage is then rapidly changed and the piezoelectric element quickly changes its size in response to the changing voltage. The inertial mass inertially resists the quick piezoelectric element movement, overcoming the clamping force and moving the clamping wafer relative to the base wafer.
In one alternate technique, the polarity of the clamping voltage can be quickly switched at the time the voltage to the piezoelectric element is rapidly changed. This helps avoid charge build up, and avoids compression of the insulating layer. The cycle is repeated to effect precise, incremental linear motion. The motion of the linear actuator can also be easily reversed.
The voltage on the electrostatic clamp and the expandable member is controlled to provide appropriate frequency, amplitude and waveform shapes. Frequencies are adjustable and preferably the frequency of the voltage waveform applied to the electrostatic clamp is some factor less than the voltage frequency applied to the expandable member. Voltage amplitude is adjustable to provide control of actuator output force. The waveform shape of the voltage applied to the clamp and the voltage applied to the expandable member is controllable to control actuator output force. An additional phase angle difference control provides control over the phase angle difference between the clamp voltage waveform and the expandable member voltage waveform.
A double clamping embodiment of the actuator is provided that uses semiconductor technology to provide a double clamping linear actuator that has useable force levels and repeatable increments regardless of the spatial orientation of the actuator.